An increasingly popular process for forming composites is by compression molding or stamping a moldable sheet of a thermoplastic resin reinforced with fibers such as a glass fiber mat, often referred to as glass mat thermoplastics or "GMT". These composite sheets can be used to form articles such as automobile components and housings for computers.
An example of a commercially successful GMT sheet is the AZDEL.RTM. moldable composite sheet which is formed by extruding layers of polypropylene resin sheet with needled mats of continuous glass fiber strand. The AZDEL.RTM. composite sheet is commercially available from Azdel, Inc. of Shelby, N.C.
For reinforcing a resin matrix material, U.S. Pat. Nos. 3,664,909, 3,713,962 and 3,850,723 disclose fibrous mats of unstranded filaments which can be layered with reinforcing mats of fiber strands.
U.S. Pat. Nos. 4,948,661, 5,011,737, 5,071,608 and 5,098,624 disclose fiber reinforced thermoplastic molded products produced by intimately blending reinforcing glass fibers and thermoplastic fibers into a web and heating the web to the melting point of the thermoplastic fibers while applying pressure to the web to press the web into a consolidated structure.
U.S. Pat. No. 4,112,174 and 4,129,674 disclose a carrier mat useful in the manufacture of roofing materials which includes a web of randomly dispersed monofilament glass fibers, elongated glass fiber bundles and a binder such as urea-formaldehyde for consolidating the mat.
U.S. Pat. Nos. 5,108,678 and 5,194,462 disclose a fiber reinforced plastic sheet having a gradient layer structure wherein less opened strands are concentrated in the lower layers and more opened strands are concentrated in the upper layers of the structure.
There is a long-felt need in the industry for a mat for applications such as GMT which can be readily impregnated by the thermoplastic matrix material, which has good uniformity of glass distribution and flow into complex mold shapes and which provides a composite having good surface smoothness and mechanical properties, such as strength and stiffness.